This invention relates to a grid for use in electron discharge devices, and particularly to a nickel mesh grid which has been coated with a layer of chromium.
A vidicon tube is one form of an electron discharge device. In the vidicon tube, an optical input is converted into an electrical output. In one such device, one surface of a photoconductive target is exposed to the optical input. The optical input is transformed into an electrical output by exposing the opposing surface of the photoconductive target to a scanning electron beam. Generally, a mesh grid is disposed between the photoconductive target and an electron gun which provides the scanning electron beam. One purpose of this grid is to ensure that the scanning electron beam possesses the proper energy, e.g., a few volts, when it strikes the photoconductive target. Another purpose of this grid is to ensure that the scanning electron beam strikes the photoconductive target at the proper locations.
Most of the grids used in these devices are copper meshes which have been prepared by electroforming onto a master pattern which has been etched on a glass plate. After the electroforming process, the mesh is mounted in a supporting structure and fired in hydrogen. This is done to make the mesh taut and to outgas the mesh so as to make it suitable for use in a vacuum tube. However, one problem associated with the use of the copper mesh grid is that, during firing, the structure stretches taut due to continued grain growth. This stretching results in a mesh which is not as strong as desirable. Indeed, in some instances, especially after long or repeated heating periods, the copper mesh may even collapse. It should be noted that most particle matter present on the mesh before firing result in mesh blemishes which make the mesh unusable for satisfactory tube operation. Repeated firing, or extended firing periods, are often required in order to reduce the magnitude of this problem. Also, the use of copper mesh grids has been plagued by continuous production yield fluctuations in the electroforming process.
Although the use of a nickel mesh grid would appear to be an alternative to the use of copper, this has not occurred. One problem is that, after being air baked and hydrogen fired, the mesh is often insufficiently taut and may exhibit a relatively high secondary emission coefficient (.delta.) so as to make the mesh grid unusable in several vidicon tubes. The secondary emission problem can be substantially reduced by coating the mesh with a layer of chromium as disclosed in U.S. Pat. No. 3,136,916, entitled, "Image Orthicon Tube Having Specially Coated Decelerating Field Electrode", issued June 9, 1964 to D. Schaefer. However, even when so coated, the mesh is insufficiently taut with relatively low resonant frequencies, i.e., below 3000 hertz. These meshes frequently result in microphonics in tubes, i.e., translation of vibration or shock into an electrical signal with measurable amplitude and duration.
Thus, it would be desirable to develop a taut mesh grid that can be mass produced with high yields without creating secondary emission problems.